On Mon, Oct 29, 2018 at 11:24:42AM +0000, Ran Rozenstein wrote:
> Hi Paul and all,
>
> > -----Original Message-----
> > From: [email protected] [mailto:linux-kernel-
> > [email protected]] On Behalf Of Paul E. McKenney
> > Sent: Thursday, August 30, 2018 01:21
> > To: [email protected]
> > Cc: [email protected]; [email protected]; [email protected];
> > [email protected]; [email protected];
> > [email protected]; [email protected]; [email protected];
> > [email protected]; [email protected]; [email protected];
> > [email protected]; [email protected]; [email protected]; Paul E.
> > McKenney <[email protected]>
> > Subject: [PATCH tip/core/rcu 02/19] rcu: Defer reporting RCU-preempt
> > quiescent states when disabled
> >
> > This commit defers reporting of RCU-preempt quiescent states at
> > rcu_read_unlock_special() time when any of interrupts, softirq, or
> > preemption are disabled. These deferred quiescent states are reported at a
> > later RCU_SOFTIRQ, context switch, idle entry, or CPU-hotplug offline
> > operation. Of course, if another RCU read-side critical section has
> > started in
> > the meantime, the reporting of the quiescent state will be further deferred.
> >
> > This also means that disabling preemption, interrupts, and/or softirqs will
> > act
> > as an RCU-preempt read-side critical section.
> > This is enforced by checking preempt_count() as needed.
> >
> > Some special cases must be handled on an ad-hoc basis, for example,
> > context switch is a quiescent state even though both the scheduler and
> > do_exit() disable preemption. In these cases, additional calls to
> > rcu_preempt_deferred_qs() override the preemption disabling. Similar logic
> > overrides disabled interrupts in rcu_preempt_check_callbacks() because in
> > this case the quiescent state happened just before the corresponding
> > scheduling-clock interrupt.
> >
> > In theory, this change lifts a long-standing restriction that required that
> > if
> > interrupts were disabled across a call to rcu_read_unlock() that the
> > matching
> > rcu_read_lock() also be contained within that interrupts-disabled region of
> > code. Because the reporting of the corresponding RCU-preempt quiescent
> > state is now deferred until after interrupts have been enabled, it is no
> > longer
> > possible for this situation to result in deadlocks involving the scheduler's
> > runqueue and priority-inheritance locks. This may allow some code
> > simplification that might reduce interrupt latency a bit. Unfortunately, in
> > practice this would also defer deboosting a low-priority task that had been
> > subjected to RCU priority boosting, so real-time-response considerations
> > might well force this restriction to remain in place.
> >
> > Because RCU-preempt grace periods are now blocked not only by RCU read-
> > side critical sections, but also by disabling of interrupts, preemption, and
> > softirqs, it will be possible to eliminate RCU-bh and RCU-sched in favor of
> > RCU-preempt in CONFIG_PREEMPT=y kernels. This may require some
> > additional plumbing to provide the network denial-of-service guarantees
> > that have been traditionally provided by RCU-bh. Once these are in place,
> > CONFIG_PREEMPT=n kernels will be able to fold RCU-bh into RCU-sched.
> > This would mean that all kernels would have but one flavor of RCU, which
> > would open the door to significant code cleanup.
> >
> > Moving to a single flavor of RCU would also have the beneficial effect of
> > reducing the NOCB kthreads by at least a factor of two.
> >
> > Signed-off-by: Paul E. McKenney <[email protected]> [ paulmck:
> > Apply rcu_read_unlock_special() preempt_count() feedback
> > from Joel Fernandes. ]
> > [ paulmck: Adjust rcu_eqs_enter() call to rcu_preempt_deferred_qs() in
> > response to bug reports from kbuild test robot. ] [ paulmck: Fix bug
> > located
> > by kbuild test robot involving recursion
> > via rcu_preempt_deferred_qs(). ]
> > ---
> > .../RCU/Design/Requirements/Requirements.html | 50 +++---
> > include/linux/rcutiny.h | 5 +
> > kernel/rcu/tree.c | 9 ++
> > kernel/rcu/tree.h | 3 +
> > kernel/rcu/tree_exp.h | 71 +++++++--
> > kernel/rcu/tree_plugin.h | 144 +++++++++++++-----
> > 6 files changed, 205 insertions(+), 77 deletions(-)
> >
>
> We started seeing the trace below in our regression system, after I bisected
> I found this is the offending commit.
> This appears immediately on boot.
> Please let me know if you need any additional details.
Interesting. Here is the offending function:
static void rcu_preempt_deferred_qs(struct task_struct *t)
{
unsigned long flags;
bool couldrecurse = t->rcu_read_lock_nesting >= 0;
if (!rcu_preempt_need_deferred_qs(t))
return;
if (couldrecurse)
t->rcu_read_lock_nesting -= INT_MIN;
local_irq_save(flags);
rcu_preempt_deferred_qs_irqrestore(t, flags);
if (couldrecurse)
t->rcu_read_lock_nesting += INT_MIN;
}
Using twos-complement arithmetic (which the kernel build gcc arguments
enforce, last I checked) this does work. But as UBSAN says, subtracting
INT_MIN is unconditionally undefined behavior according to the C standard.
Good catch!!!
So how do I make the above code not simply function, but rather meet
the C standard?
One approach to add INT_MIN going in, then add INT_MAX and then add 1
coming out.
Another approach is to sacrifice the INT_MAX value (should be plenty
safe), thus subtract INT_MAX going in and add INT_MAX coming out.
For consistency, I suppose that I should change the INT_MIN in
__rcu_read_unlock() to -INT_MAX.
I could also leave __rcu_read_unlock() alone and XOR the top
bit of t->rcu_read_lock_nesting on entry and exit to/from
rcu_preempt_deferred_qs().
Sacrificing the INT_MIN value seems most maintainable, as in the following
patch. Thoughts?
Thanx, Paul
------------------------------------------------------------------------
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index bd8186d0f4a7..f1b40c6d36e4 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -424,7 +424,7 @@ void __rcu_read_unlock(void)
--t->rcu_read_lock_nesting;
} else {
barrier(); /* critical section before exit code. */
- t->rcu_read_lock_nesting = INT_MIN;
+ t->rcu_read_lock_nesting = -INT_MAX;
barrier(); /* assign before ->rcu_read_unlock_special load */
if (unlikely(READ_ONCE(t->rcu_read_unlock_special.s)))
rcu_read_unlock_special(t);
@@ -617,11 +617,11 @@ static void rcu_preempt_deferred_qs(struct task_struct *t)
if (!rcu_preempt_need_deferred_qs(t))
return;
if (couldrecurse)
- t->rcu_read_lock_nesting -= INT_MIN;
+ t->rcu_read_lock_nesting -= INT_MAX;
local_irq_save(flags);
rcu_preempt_deferred_qs_irqrestore(t, flags);
if (couldrecurse)
- t->rcu_read_lock_nesting += INT_MIN;
+ t->rcu_read_lock_nesting += INT_MAX;
}
/*
